Combustion-based emission reduction method and system

ABSTRACT

In a combustion apparatus having multiple primary fuel inputs and a flue gas exhaust, a method for controlling combustion of a fuel in the combustion chamber in which a concentration of at least one flue gas component indicative of fuel/oxidant ratio in flue gases is measured at a plurality of locations proximate the flue gas exhaust, each location corresponding to one of the primary fuel inputs. An overall average concentration of the at least one flue gas component is then determined, based upon which a delta value, the difference between the overall average and the flue gas component concentration at each location, for the at least one flue gas component at each of the locations is determined. Based upon the delta values obtained, the fuel input rate is adjusted as necessary for each of the primary fuel inputs, such that the delta value is either reduced or increased to zero.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method and system for controllingcombustion-based emissions from solid-, liquid- or gaseous-fuel firedcombustors. More particularly, this invention relates to combustorshaving multiple fuel inputs such as grate-fired spreader-stokers, whichtypically employ multiple solid fuel feeders, and furnaces havingmultiple liquid- and/or gaseous-fuel fired burners.

[0003] 2. Description of Related Art

[0004] Furnaces having multiple primary fuel inputs invariably sufferfrom unbalanced primary fuel distribution. This can lead to non-uniformstoichiometry in the primary combustion regions or zones, which, inturn, results in increases in the formation of NO_(x), CO and, in thecase of solid fuel combustors, particulates. In addition, uneven primaryfuel distribution in stoker systems leads to grate problems includingpiling and rat-holing.

[0005] There are a substantial number of control schemes for controllingand optimizing combustion systems known to those skilled in the art.Exemplary of such control schemes is taught by U.S. Pat. No. 4,592,289to Pershing et al. in which pollutant emissions, including particulateemissions, from spreader-stoker-fired furnaces are reduced bycontrolling the stoichiometric ratio of oxygen to combustible materialin different regions of the furnace, which control is accomplished bycontrolling the amount of air injected into different regions of thefurnace. Indeed, controlling the combustion airflows within the furnaceis a primary element of known methods and devices for reducing emissionsfrom furnaces.

[0006] Numerous methods and apparatuses also are known for controllingNO_(x) formation in stokers, including fuel reburn, flue gasrecirculation and amine enhanced fuel reburn. U.S. Pat. No. 5,937,772 toKhinkis et al. teaches a process for combustion in which a combustiblematerial such as coal, municipal solids wastes, wood, wood waste, refusederived fuels, biomass and sludge is introduced into a stoker andburned, creating a primary combustion zone or region. A mixture of fluegases and flyash having an organic content is injected into the stokerdownstream of the primary combustion region to create anoxygen-deficient reburn zone, thereby reducing the NO_(x) emissions fromthe stoker. See also U.S. Pat. No. 5,205,227 to Khinkis et al. whichteaches the injection of a gaseous fuel, e.g. natural gas, into a stokerdownstream of the primary combustion region resulting in a reduction inNO_(x) emissions compared to conventional stokers not employing fuelreburn.

[0007] Notwithstanding the substantial strides that have been madetoward reducing flue gas emissions from stokers and other fossil fuelfired furnaces, non-uniformity of the combustion, particularly instokers and furnaces employing multiple primary fuel inputs, whichnon-uniformity can result in increased emissions, remains a concern.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is one object of this invention to provide amethod and apparatus for controlling pollutant emissions from solid-,liquid-, or gaseous-fuel fired combustors.

[0009] It is one object of this invention to provide a method andapparatus for controlling pollutant emissions from solid-, liquid-, orgaseous-fuel fired combustors having multiple primary fuel inputs, suchas multi-feeder stokers.

[0010] It is yet another object of this invention to provide a methodand apparatus for improving the uniformity of combustion within solid-,liquid-, or gaseous-fuel fired combustors.

[0011] It is still a further object of this invention to provide amethod and apparatus for controlling pollutant emissions from solid-,liquid-, or gaseous-fuel fired combustors having multiple primary fuelinputs by controlling individually the amount of primary fuel introducedinto the combustors through each of the primary fuel inputs so as toregulate primary fuel distribution within the combustor.

[0012] These and other objects are addressed in a combustor havingmultiple primary fuel inputs by a method and apparatus in which theoccurrence of non-uniform combustion is detected, the location of thenon-uniform combustion is determined and the fuel distribution isautomatically manipulated to correct the combustion irregularity. Usingthe method and apparatus of this invention results in primary combustionstoichiometry in the combustor that is more uniform, more stable and atcloser to optimum value than is possible with conventional combustorswith multiple primary fuel inputs, which, in turn, results in thereduction of pollutant emissions from the combustor. In addition, whenemployed in combination with conventional emission control technology,such as low-NO_(x) burners and reburn technology, greater reductions inNO_(x), CO and particulates (PM₁₀ and PM_(2.5)) are achieved than arepossible with conventional technology alone.

[0013] The method and apparatus of this invention are based upon thediscovery that there is a direct correlation between the concentrationof pollutants at different locations in the flue gases proximate to theflue gas exhaust from the combustor and the distribution of primary fuelinput to the combustor. Tests conducted on a stoker boiler havingmultiple solid fuel feeders established the existence of a correlationbetween local grate combustion conditions and respective localizedcarbon monoxide and/or combustible levels in the flue gases exhaustedfrom the combustor. The method and apparatus of this invention utilizesthis correlation in a regulatory control scheme in which compositionanalysis of the flue gas is used as feedback for obtaining uniformcombustion in the primary combustion regions of the stoker. Althoughother components in the flue gases could possibly be used, for example,elements introduced for this purpose through the individual primary fuelinputs, carbon monoxide or combustible analysis in the flue gas providesan acceptable feedback signal. Flue gas oxygen analysis may be used formonitoring purposes.

[0014] In accordance with the method of this invention for controllingcombustion of a primary fuel in a combustion chamber having a pluralityof primary combustion regions corresponding to multiple primary fuelinputs and a flue gas exhaust, a concentration of at least one flue gascomponent that is indicative of fuel/oxidant ratio in the flue gasesgenerated by the combustion is measured at a plurality of locationsproximate the flue gas exhaust, resulting in a plurality of measuredconcentrations. Each of the locations at which the measurements are madecorresponds to one of the primary combustion regions. To dampenmeasurement noise and reduce unnecessary control fluctuations, thereal-time measurements of the at least one flue gas component areprocessed into a localized rolling average signal. An overall averageconcentration of the at least one flue gas component is determined fromthe plurality of real-time measured concentrations. Thereafter, a deltavalue for the at least one flue gas component at each of the locationsis determined. Delta values are the result of the difference between theat least one flue gas component concentration at each location and theoverall average concentration of the at least one flue gas component.Positive delta values indicate excess fuel while negative delta valuesindicate a shortage of fuel. Based upon the determination of deltavalues, the fuel input rate for each of the primary combustion regionsis adjusted, as necessary, either by increasing or decreasing the fuelinput rate, until the delta value is reduced or increased to zero.

[0015] The apparatus of this invention, in accordance with oneembodiment, for controlling the combustion in a combustion apparatuscomprising a combustion chamber having a plurality of primary combustionregions and a flue gas exhaust, where each of the primary combustionregions has a corresponding primary fuel input, comprises a combustioncontrol system comprising a plurality of flue gas sensors disposedwithin the combustion chamber at a plurality of sensor locationsproximate the flue gas exhaust. Each of the flue gas sensors is adaptedto measure an amount of at least one flue gas component that isindicative of fuel/oxidant ratio. A data processor is operably connectedto the flue gas sensors and is adapted to determine an average amount ofthe at least one flue gas component measured by the plurality of fluegas sensors. The data processor comprises a plurality of comparatorblocks adapted to generate a delta value for the at least one flue gascomponent for each of the sensor locations. At least one fuel inputcontroller is operably connected to the processor and to a fuel supplyto each of the primary combustion regions. The at least one fuel inputis adapted to control fuel input to each of the primary combustionregions based upon the delta values.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other objects and features of this invention will bebetter understood from the following detailed description taken inconjunction with the drawings wherein:

[0017]FIG. 1 is a cross-sectional view of a grate-fired spreader-stokersystem with a combustion-based emissions reduction system in accordancewith one embodiment of this invention;

[0018]FIG. 2 is a cross-sectional view of the grate-firedspreader-stoker shown in FIG. 1 taken along the line II-II;

[0019]FIG. 3 is a diagram showing the effect of changes in stoker feedconditions on combustibles exiting from the stoker; and

[0020]FIG. 4 is a detailed schematic diagram of the control system inaccordance with one embodiment of this invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0021] The invention claimed herein is a method and system forcombustion-based emissions reduction from spreader-stokers, grate firedcombustors and furnaces all of which employ multiple primary fuel inputmeans. In the case of stokers, the primary fuel input means are in theform of multiple feeders. Other furnaces or combustors to which themethod and system of this invention may be applied are furnaces andcombustors having multiple solid-, liquid- and/or gaseous-fired primaryfuel burners. Although described in the context of a grate-fired stoker,the method and system of this invention are applicable to any furnacesystem employing multiple primary fuel inputs and no intention to limitthe scope of this invention to grate-fired stokers should be inferred.

[0022]FIGS. 1 and 2 show a grate-fired spreader stoker comprising acombustion-based emissions reduction system in accordance with oneembodiment of this invention. Stoker 10 comprises a stoker grate 11disposed in the bottom portion on top of which is disposed a solid fuel13, which is introduced into the combustion chamber 30 of stoker 10through solid fuel feeders 12. Primary combustion air for combustion ofthe solid fuel 13 is provided to the stoker grate 11 as undergrate air15. The rate of solid fuel introduced through solid fuel feeders 12 iscontrolled by fuel rate controller 31, which comprises a variable speedmotor 28 and a variable speed driver 29 operably connected to variablespeed motor 28. Stoker 10 further comprises a flue gas exhaust 21through which the flue gases generated by the combustion of the solidfuel 13 are exhausted from stoker 10. Ash which is generated as theresult of the combustion of the solid fuel 13 is discharged from stoker10 through ash discharge 14.

[0023] As shown in FIG. 2, combustion chamber 30 comprises a pluralityof primary combustion regions 18, 19 and 20 disposed proximate stokergrate 11. Each primary combustion region 18, 19 and 20 corresponds toone of the solid fuel feeders 12. As a result, the combustion of thesolid fuel 13 within each of the primary combustion regions 18, 19 and20 can be modified by altering the fuel input rate of solid fuel 13introduced through corresponding solid fuel feeders 12.

[0024] Combustion-based emissions reductions are performed by anadvanced control logic system in accordance with one embodiment of thisinvention comprising process instrumentation in the form of a pluralityof flue gas component sensors 26 operably connected to flue gascomponent analyzers 25 suitable for analyzing flue gas composition. Fluegas component sensors 26 are disposed at a plurality of flue gascomponent locations 22, 23, 24, which locations are proximate to fluegas exhaust 21. Localized measurements are made of flue gas componentswhich, based upon the amounts present in the flue gas, can be used asindicators of the fuel/air ratio of the primary fuel and air employed inthe combustion occurring in the primary combustion regions. Inaccordance with one embodiment of this invention, localized measurementsof flue gas oxygen (O₂), carbon monoxide (CO), nitrogen oxides (NO_(x))and/or combustibles are obtained in real time and serve as the primaryinputs to the control logic system.

[0025] In addition to process instrumentation, the advanced controllogic system of this invention comprises a data processor, identified inFIG. 1 by elements labeled “AY”. The data processor processes theoxygen, carbon monoxide and combustibles real time measurements into arolling average signal (AVG in FIG. 1), which dampens measurement noiseand reduces unnecessary control fluctuations. The localized rollingaverage signals for the different gas components are compiled and anoverall arithmetic average of each flue gas component is calculated.

[0026] Using dedicated comparator blocks, the advanced control logicsystem generates delta values for each or selected flue gas components.Delta values represent the difference between the localized measurementsof concentration levels for the flue gas components and the overall,arithmetically derived, concentration level for the flue gas components.Given a system utilizing a combustible concentration component, positivecomparator delta values indicate excess fuel while negative comparatorvalues indicate a shortage of fuel within each of the primary combustionregions. If the delta value from each respective comparator block isequal to zero, then combustion stoichiometry for the primary fuel andair is considered to be uniform. If, however, a delta value for acomparator block is not zero, the advanced control logic systemselectively adjusts the local rate at which fuel is supplied to thedifferent primary combustion regions 18, 19 and 20 such that comparatordelta values of about zero are restored. The advanced control logicsystem of this invention is enabled only during periods when the deltavalues contain at least one negative value or one positive value. If allthe delta values are positive, then greater combustion air flow isrequired; likewise, if all the delta values are negative, lesscombustion air flow is required. Under these latter two scenarios, theadvanced control logic system is disabled.

[0027] Fuel redistribution is implemented through a control bias schemeapplied to a set point control signal for each fuel input, in the caseof a stoker, each of the feeders. A positive set point bias increasesfeeder speed while a negative set point bias decreases feeder speed.Feeder speed is varied by way of variable speed driver 29, which isoperably connected to a motor 28, which, in turn, is operably connectedto a feed rate controller 31. The amount of set point bias is dependentupon the delta value, the feeder's correlation factor, K, and tuningparameters. This K factor allows for fine-tuning the control responseand is empirically determined through tests which characterize aspecific feeder's sensitivity on local stoichiometry. Aproportional-integral (PI) controller is also included in the loop foradditional tuning of the control response. As a control precaution, theadvanced control logic system of this invention includes bias limits tosafeguard against unexpected control action that might swing the boilerinto an excessive abnormal condition.

[0028] Fuel feeders in spreader-stokers comprise multiple mechanical,constant-volume screw or slat-type conveyors to ensure proper grate fuelcoverage and the proper amount of fuel to meet the thermal demands ofthe furnace. For a stoker boiler, a boiler master controller isresponsible for controlling total heat demand by regulation of fuel flowvia modulation of feeder conveyor speed as measured in revolutions perminute (rpm). Feeder tests conducted on a coal-fired spreader-stokerestablished that a 1% change in conveyor speed produced a change ofabout 3.5% in the boiler flue gas combustible concentration (based upona full scale of 5000 dppmv). During this test the flue gas oxygen wasmeasured at 2-3%. At lower oxygen levels (about 1%), feeder changesproduced a stronger response in flue gas combustibles as shown in FIG.3.

[0029] The combustion-based emissions reduction system of this inventionrelies on modulation of feeder speed (rpms) through changes in thefeeder's bias controller output value. Bias adjustments can eitherincrease or decrease feeder speeds. System control logic increases thespeed of selected feeders while decreasing the speed of other feeders,all the while maintaining total feeder revolutions per minute constant.This response eliminates interferences with a stoker's thermal heatinput as set by the boiler master controller and steady steam productionis maintained. Stoker primary combustion regions are also maintained atoptimum stoichiometry to achieve the greatest reduction in NO_(x) andother regulated emissions.

[0030] A more detailed combustion-based emissions reduction controlsystem is shown in FIG. 4. This illustration shows a controlconfiguration for a spreader-stoker boiler having five feeder units andthree boiler exit flue gas combustible analyzers. The system comprisessimilar control blocks and control functions associated withcombustion-based emissions reduction as previously described. In thisexample, additional control elements calculate simulated real-time andaverage gas compositions to implement bias control of #2 and #4 feeders.Direct measurements of boiler exit combustibles control the bias for the#1, #3 and #5 feeders. In this example, feeder rpm is changed through avariable-speed controller regulating the feeder drive motor rpm.

[0031] In accordance with one embodiment of this invention, thecombustion-based emissions reduction system of this invention is appliedto furnaces employing other advanced techniques for reducing NO_(x)emissions. One of these techniques, as shown in FIG. 1, known as fuelreburn, involves the injection of a fuel through a reburn fuel input 16downstream of the primary combustion regions and the injection ofoverfire air through overfire air input 17 downstream of the reburn fuelinput 16. The use of this invention in combination with conventionalreburn technology provides up to a 90% NO_(x) emissions reductioncompared to baseline emissions. Attributes which appear to account forthis include the ability of the method of this invention to controlstoichiometry at 0.6 to 1.0 (Control of grate combustion stoichiometryis a major variable in NO_(x) formation); improved level of combustionuniformity and combustion stoichiometry over the entire grate; reductionin the number of non-uniform occurrences; and increased rates of reburnfuel injection, up to 15 to 25% compared to conventional uses of 5 to10% thermal input due to improved grate combustion zone conditions.

[0032] In addition, the method and system of this invention areadaptable to solid, liquid or gas-fired combustors, furnaces andboilers. The logic employed by this invention continuously monitors,detects and manipulates localized fuel response to non-uniformstoichiometry as inferred from flue gas composition. Grate fueldistribution is controlled without interacting or interfering withfurnace or boiler master demand for total thermal heat input.Application of this invention to stokers results in fewer incidences ofgrate problems due to piling, rat holing and hot spots, reductions incombustor particulates, reduction in unburned carbon levels in bottomash and fly ash, boosts in boiler thermal efficiency, less maintenancedue to fouling and deposition on boiler internals, improved heat releaseprofile at the grate due to more stable and uniform combustion, andimproved depth and evenness of the ash layer on the grate.

[0033] While in the foregoing specification this invention has beendescribed in relation to certain preferred embodiments thereof, and manydetails have been set forth for purpose of illustration, it will beapparent to those skilled in the art that the invention is susceptibleto additional embodiments and that certain of the details describedherein can be varied considerably without departing from the basicprinciples of the invention.

1. In a combustion apparatus comprising a combustion chamber having aplurality of primary combustion regions and a flue gas exhaust, each ofsaid primary combustion regions having a corresponding fuel input, amethod for controlling combustion of a fuel in said combustion chambercomprising the steps of: measuring a concentration of at least one fluegas component indicative of fuel/oxidant ratio in flue gases generatedby said combustion at a plurality of locations proximate said flue gasexhaust, resulting in a plurality of measured concentrations, each ofsaid locations corresponding to one of said primary combustion regions;determining an average concentration of said at least one flue gascomponent from said plurality of measured concentrations; determining adelta value for said at least one flue gas component at each of saidlocations; and adjusting a fuel input rate as necessary for each of saidprimary combustion regions, whereby said delta value is one of reducedand increased to zero.
 2. A method in accordance with claim 1, whereinsaid at least one flue gas component is selected from the groupconsisting of CO, O₂, NO_(x), combustibles and mixtures thereof.
 3. Amethod in accordance with claim 1, wherein said combustion apparatuscomprises a combustion system selected from the group consisting ofgrate-fired spreader stokers and multiple burner furnaces.
 4. A methodin accordance with claim 3, wherein said combustion apparatus is agrate-fired spreader stoker having a plurality of solid fuel feeders. 5.A method in accordance with claim 1 further comprising introducing areburn fuel into a fuel reburn region in said combustion chamberdisposed between said plurality of primary combustion regions and saidflue gas exhaust.
 6. A method in accordance with claim 5 furthercomprising introducing overfire air into an overfire air region in saidcombustion chamber disposed between said reburn fuel region and saidflue gas exhaust.
 7. In a combustion apparatus comprising a combustionchamber having a plurality of primary combustion regions and a flue gasexhaust, each of said primary combustion regions having a correspondingfuel input, the improvement comprising: a combustion control systemcomprising a plurality of flue gas sensors disposed within saidcombustion chamber at a plurality of sensor locations proximate saidflue gas exhaust, each of said flue gas sensors adapted to measure anamount of at least one flue gas component indicative of fuel/oxidantratio; a data processor operably connected to said flue gas sensors andadapted to determine an average amount of said at least one flue gascomponent measured by said plurality of flue gas sensors, said dataprocessor comprising a plurality of comparator blocks adapted togenerate a delta value for said at least one flue gas component for eachof said sensor locations; and at least one fuel input controlleroperably connected to said processor and a fuel supply to each of saidprimary combustion regions, said at least one fuel input adapted tocontrol fuel input to each of said primary combustion regions based uponsaid delta values.
 8. An apparatus in accordance with claim 7, whereinsaid fuel supply comprises a plurality of solid fuel feeders of agrate-fired spreader stoker, each of said solid fuel feeders providingsolid fuel to a corresponding said primary combustion region.
 9. Anapparatus in accordance with claim 8, wherein said fuel input controllercomprises a speed controller corresponding to each of said solid fuelfeeders and adapted to control a speed of said corresponding solid fuelfeeder based upon output signals from said processor.
 10. An apparatusin accordance with claim 7, wherein said fuel supply comprises aplurality of fossil fuel burners, each of said fossil fuel burnersadapted to deliver fuel to one of said primary combustion regions. 11.An apparatus in accordance with claim 10, wherein said fuel inputcontroller is adapted to independently control each of said fossil fuelburners.
 12. An apparatus in accordance with claim 10, wherein saidfossil fuel burners are fired with one of a liquid fossil fuel and agaseous fossil fuel.
 13. An apparatus in accordance with claim 7,wherein said at least one flue gas component is selected from the groupconsisting of CO, O₂, NO_(x), combustibles and mixtures thereof.
 14. Anapparatus in accordance with claim 8 further comprising at least onereburn fuel burner adapted to introduce a reburn fuel into saidcombustion chamber in a fuel reburn region disposed between said primarycombustion regions and said flue gas exhaust.
 15. An apparatus inaccordance with claim 14 further comprising at least one overfire airnozzle adapted to introduce overfire air into said combustion chamber inan overfire air region disposed between said fuel reburn region and saidflue gas exhaust.